Polyurethanes staining methods

Some aspects of morphology can be observed directly by transmission electron microscopy of stained and ultramicrotomed thin sections. The most successful staining method, developed by Kato, makes use of osmium tetroxide, which attacks the double bonds in diene type polymers. The OSO4 staining technique can also be used with other active groups, such as polyurethanes.Many saturated or nonreactive polymers are not easily studied by transmission electron microscopy, unfortunately, because they cannot be stained. Other aspects of morphology, such as phase continuity and interface characteristics, are best determined by combining chemical and dynamic mechanical spectroscopy methods with electron microscopy. 

Early results with cryomicrotomes were described by Cobbold and Mendelson [80]. Polyurethane elastomer, a blend of crystalline and noncrystalline polymers, showed spherulitic textures after sectioning at about -70°C. Injection molded polypropylene (PP) was also sectioned at about -70°C, while polytetrafluoroethylene (PTFE) was sectioned at much lower temperatures. The authors concluded that the technique, though difficult, had potential. Extruded styrene-butadiene-styrene (SBS) copolymer was prepared by cryosectioning with a diamond knife in liquid air at —85 to —115°C, followed by osmium tetroxide vapor staining for one hour [81]. This method revealed the alternating sequence of the polystyrene and polybutadiene lamellae. Odell et al. [82] prepared extruded triblock copolymer by first chemically hardening the polybutadiene, with osmium tetroxide, followed by cryoultramicrotomy to produce 30 nm thick sections which showed fine structure details. Parallel polystyrene rods were observed in the SBS copolymer. Ultramicrotomy and selective staining with osmium tetroxide was also used in the preparation of a binary blend of PP and thermoplastic rubber [83]. 

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